The object of the proposed research is to determine the respiratory relationships between external loading and power output of the human breathing system and to infer from the results some of the properties of the internal breathing mechanisms. The basic measurement will be to have a subject exert an inspiratory effort against a closed shutter and to measure flow and pressure at the mouth immediately following the abrupt opening of the shutter, distal to which will be located an external resistance. The following will be treated as parameters: initial lung volume, initial inspiratory pressure, and magnitude of external resistance. Pressure-flow curves will be constructed and external power of breathing will be computed. From this information an apparent impedance of the breathing system will be inferred and conditions for maximal power output will be determined.